1. Field of the Invention
This invention teaches a moving-coil, optical-fiber accelerometer for measuring accelerations such as may be due to particle motions of the earth.
2. Discussion of the Prior Art
Optical-fiber sensors for measuring pressure, strain, temperature and other physical quantities are known. Generally such sensors depend upon the fact that the quantity to be measured somehow changes certain optical characteristics of a cladded glass or silica fiber. For example, application of a transient pressure field to a fiber changes the length and/or index of refraction of the fiber as a function of the change in pressure with respect to time. A beam of coherent radiation is launched into a measurement fiber and, in parallel, into a shielded reference fiber. The two emergent beams are recombined, usually interferometrically. The transient pressure field modulates the beams propagating through the measurement fiber to cause a phase shift of the modulated beam relative to the reference beam. The resulting pattern of interference fringes is probed by a photodetector to convert the observed phase shifts to an electrical anlog signal. A typical pressure responsive sensor is disclosed in U.S. Pat. No. 4,320,475 to Le Clerc et al.
An accelerometer can be created by applying a pressure to an optical fiber by means of an inertia mass. The sensitivity of such accelerometers is enhanced by micro-bending the fiber as between a fixed and a moveable corrugated grid. The output beam is intensity modulated. One such sensor is taught by U.S. Pat. No. 4,530,078 to Logakos et al.
Known optical sensors tend to be somewhat complex and delicate, particularly those that depend upon phase shift modulation. The interferometer/photodetector combination prefers a benign laboratory environment over the harsh conditions in the field.
Optical-fiber systems in general, including seismic sensor systems, suffer from induced polarization effects due to stresses and bends in the fibers. It is necessary to control the state of polarization of the radidation propagating through the system. To that end, polarization controllers are provided. One type of polarization controller is disclosed in U.S. Pat. No. 4,389,090 which is incorporated herein by reference.
Briefly the controller depends upon the photo-elastic effect for its operation. An optical fiber strand is bent into a loop of one or more turns having a relatively small radius. Photoelasticity causes the normally isotropic fiber to become birefringent under stress. Upon rotation of the loop, about an axis in the plane of the loop, the axis of birefringence is rotated, thus producing a change in the state of polarization of the radiation propagating through the polarization controller. In use, the polarization controller is inserted into the optical system under consideration. The optical-fiber loop is rotated to a desired angle and is locked in place for the duration of the progress of the experiment. Usually a two- or three-unit polarization controller is provided so as to control any state of polarization.